1. FIELD OF THE INVENTION: The invention relates to a method and apparatus for controlling the firing of a well perforating gun so that the gun is fired under optimum pressure conditions surrounding the gun.
2. SUMMARY OF THE PRIOR ART: The perforating of subterranean wells in a so-called underbalanced condition is a technique that has been widely heretofore employed. Briefly, such technique involves the utilization of a tubing conveyed perforating gun which, together with a packer located on the tubing string above the perforating gun is lowered to the desired position in the well casing. The packer is then set and the fluid pressure existing around the perforating gun is then reduced to a desired level, substantially below hydrostatic fluid pressure. Such reduced fluid pressure may be achieved by recirculating a light density fluid into the tubing string, by swabbing the tubing, or by running the tubing string into the well in a dry condition and incorporating a normally closed valve in the lower portions of the tubing string, then opening such valve after the packer has been set.
There is, of course, the accompanying problem of insuring that the perforating gun is not prematurely discharged during its insertion into the well. One prior art approach for solving this problem is to utilize a firing mechanism which incorporates a selected time delay. Once this firing mechanism has been placed in operation, the operator knows the amount of time available to him to achieve the desired fluid pressure conditions surrounding the perforating gun in order to fire at the optimum underbalanced condition.
It often happens that unforseen circumstances prevent the operator from achieving the desired underbalanced pressure condition prior to the end of the time delay and the firing of the perforating gun occurs under less than optimum conditions.
There is a need, therefore, for a firing control system for a well perforating gun which will insure against premature firing of the perforating gun during insertion of the gun into the well, or removal of the gun in the event that unforseen conditions prevent the firing of the gun. Additionally, the desired firing control system should not effect the firing of the gun until the optimum underbalanced fluid pressure conditions surrounding the gun have been attained, regardless of the amount of time required to effect such attainment.